1. Field of the Invention
The present invention relates to an image heating apparatus used in an image forming apparatus such as a copying machine, printer, facsimile apparatus, or multifunction peripheral having a variety of these functions.
2. Description of the Related Art
There is demand for a fixing apparatus capable of increasing temperature from a low temperature to a fixable temperature for shortening the time required to form an image after a power supply is turned on. One method of realizing the demand is supplying as much electric power as possible to the fixing apparatus in increasing temperature. However, the current flowing in the fixing apparatus is not allowed to exceed the rated current of a commercial power supply, so that the electric power to be supplied is inevitably limited.
The current flowing in increasing the temperature of the fixing apparatus depends on the resistance of a heater for heating a fixing member (heating member). As an example, if a ceramic heater is used, the resistance has a tolerance of approximately ±10%. At this point, the electric power supplied to the fixing apparatus including a resistance with a tolerance of +10% is reduced by approximately 20% of the electric power supplied to the fixing apparatus including a resistance with a tolerance of −10% if the applied voltage is the same, so that the fixing apparatus including a resistance with a tolerance of +10% consumes time for increasing temperature. On the other hand, increasing voltage applied to the heater so that the fixing apparatus including a resistance with a tolerance of +10% can increase temperature in a short time causes current flowing in the fixing apparatus including a resistance with a tolerance of −10% to exceed the rated current.
In situations where the voltage of a commercial power supply is unstable, variations in voltage affect current flowing in the fixing apparatus. If the voltage of a commercial power supply varies by ±10%, the electric power supplied to the fixing apparatus is reduced by approximately 40% at a maximum, in combination with a tolerance of ±10% of the resistance in the heater.
In an apparatus discussed in Japanese Patent Application Laid-Open No. 2006-113364, a heater is energized and the supplied electric power is estimated based on variations in the temperature of the heater to be energized to adjust the voltage applied to the heater.
In the apparatus discussed in Japanese Patent Application Laid-Open No. 2006-113364, however, a part of the electric power supplied, to the heater is used for increasing the temperature of the heater and the other parts thereof turn into heat transmitted to a pressure roller in contact with the fixing member. The quantity of heat transmitted to the pressure roller is varied with the influence of temperature and hardness of the pressure roller. Therefore, it is difficult to accurately measure the supplied electric power by the method discussed in Japanese Patent Application Laid-Open No. 2006-113364.
Low accuracy of detecting electric power is attributed to that the electric power supplied in heating is obliged to be reduced. Suppose that the electric power used in the fixing apparatus is 1000 W, for example. If a detection error of electric power is ±5%, the electric power is controlled to 952 W or less in design to guarantee that the electric power does not exceed 1000 W even if the electric power is shifted by +48 W due to a detection error of +5%. However, a detection error of electric power is as high as ±20%, the electric power needs to be controlled to 833 W or less to fall within 1000 W, or less if the electric power is shifted by +167 W due to a detection error of +20%.
A method for directly measuring the electric power supplied to the fixing apparatus and a method for measuring the temperature of the pressure roller need to specially provide a measuring instrument.